Lignin is a kind of natural phenolic polymer, which is composed of phenylpropane structural units connected by carbon carbon bond and ether bond. Lignin molecular structure contains many kinds of active groups, such as phenol hydroxyl, alcohol hydroxyl and carboxyl, which can be used in the synthesis of polymer instead of traditional petrochemical products. Chemical degradation is an effective method of lignin degradation, which is of great significance to the efficient and high value utilization of lignin resources and the sustainable development of economy and society.
In recent years, the research progress of chemical degradation of lignin and the application of degradation oligomers in polymer materials were reviewed. Chemical structure and composition of lignin
Lignin is the main component of plant body, and its chemical structure is shown in Figure 1. The content of lignin in different kinds of plants is different, but the basic structure of lignin is similar. The molecular structure of lignin usually contains three types of basic structural units: p-hydroxyphenyl (H), guaiacyl (g) and syringyl (s), and the contents of three types of structural units are different in different sources of lignin. Conifer is dominated by G-type structural units, hardwood is dominated by G-type and S-type structural units, while herb contains G-type, S-type and H-type structural units (Table 1).
The functional groups in the molecular structure of lignin mainly include hydroxyl, carbonyl, carboxyl, methoxy, etc., and the content of functional groups in different sources of lignin is different (Table 2). The type and content of functional groups determine the dispersing properties, optical properties and chemical reaction activity of lignin. Among them, phenol hydroxyl group and methoxy group have significant effect on the chemical reaction activity of lignin. This is due to the strong electronegativity of the oxygen atoms in the phenolic hydroxyl and methoxy groups, which can form a p-π conjugate system with the π electron cloud of the benzene ring, thus increasing the density of the ortho and para electron clouds of the benzene ring and enhancing the chemical reaction activity.
In addition, lignin obtained by different pretreatment methods has different characteristics in structure and physical and chemical properties (Table 3). At present, the extraction technology of lignin mainly includes sulfite method, sulfate method, soda ash method, organic solvent method and enzymatic method. Lignosulfonate, sulfate lignin, alkali lignin, organic solvent soluble lignin and enzymatic lignin can be obtained respectively. Compared with other kinds of lignin, the molecular structure of lignosulfonate and sulfate lignin contains a certain amount of sulfur, and the sulfur content of lignosulfonate is slightly higher, which mainly exists in the side chain in the form of sulfonic group, while the sulfur element in sulfate lignin mainly exists in the form of sulfhydryl group.
2 chemical degradation of lignin
The degradation of lignin mainly produces vanillin, hydroxylated aromatic hydrocarbon, quinine, aliphatic acid and other chemicals, as well as low molecular weight compounds such as lignin oligomer, which can effectively improve the reaction activity, and is of great significance to promote the efficient utilization of lignin. The chemical degradation methods of lignin mainly include reduction degradation, oxidation degradation, hydrothermal degradation and ionic liquid assisted degradation.
2.1 reductive degradation
Reductive degradation is a degradation method in which ether bond (such as β - O-4 bond, α - O-4 bond, 4-o-5 bond, etc.) and C-C bond (such as β - β bond, β - 5 bond, etc.) in lignin break under the action of hydrogen source and catalyst. The degradation products are mainly phenol compounds and lignin oligomers.
In the process of lignin degradation, hydrogen sources mainly include hydrogen and organic solvents. Hydrogen is the most common hydrogen source. The presence of hydrogen helps to inhibit the formation of coke and change the degradation efficiency and product composition of lignin. Therefore, hydrogen is often used as the hydrogen source in the study of reducing lignin. Because the selection of catalyst plays a key role in the composition and yield of degradation products, it is necessary to study the effect of catalyst on degradation reaction when hydrogen is used as hydrogen source. Galkin et al. Investigated the catalytic effect of Pd / C catalyst on the degradation of birch, poplar, spruce and pine. It was found that the degradation effect of lignin was related to its natural structure. Under the same conditions, the degradation degree of four lignin (phenol monomer yield) was positively related to the content of β - O-4 bond, which was birch & gt; poplar & gt; spruce & gt; pine in turn. The results show that β - O-4 bond is the key factor affecting the degradation of lignin, and it is an important way to improve the degradation of lignin to select the appropriate catalyst to catalyze the fracture of β - O-4 bond. Zhang et al. Used CuO / SO42 - / ZrO2 as catalyst to reduce and degrade alkali lignin, breaking β - O-4 bond in lignin and methoxyl group on benzene ring to obtain phenol products. Compared with lignin raw materials, the chemical reaction activity of phenol products is enhanced and has higher oxidation resistance. Hakonen et al. Studied the effect of Pd / C catalyst on lignin degradation. The results show that the catalyst can significantly affect the composition of aromatic degradation products. Under the action of catalyst, aromatic degradation products can form long saturated side chain, while without catalyst, most degradation products show unsaturated side chain and carbonyl side chain. Li et al. Used the modified Ni / h - β (Ni / deal) catalyst to reduce and degrade lignin, and obtained a dimer oligomer with a yield of 88.6%, which can be well soluble in solvents such as petroleum ether. This is because the modified Ni / h - β (Ni / deal) catalyst contains active acid sites and hydrogen binding sites, which is helpful to improve the efficiency of catalytic reduction and degradation. G ó mez monedero et al. Studied the effect of Ru catalyst supported on MWCNTs on the degradation of lignin. The results show that the catalyst has a high selectivity for the degradation of lignin, which can break 80% of the β - O-4 bond in the molecular structure of lignin and reduce the average molecular weight from 3600u to 1900u. Liu et al. Used water / n-butanol as solvent and Ni / HZSM-5 as catalyst to degrade corncob lignin. The results show that the catalyst can effectively promote the cleavage of β - O-4 bond and C β - C γ in lignin molecules, and generate oligomers such as dimers and trimers, thus reducing the molecular weight of lignin.
In terms of catalyst selection, the composite catalytic system and the combination of chemical and physical catalytic methods are also the research focus of hydrogen reduction degradation of lignin. Shu et al. Studied the catalytic degradation of lignin by metal chloride composite Pd / C catalyst, and found that Pd / C and metal chloride have synergistic catalytic effect, and can selectively generate phenolic compounds. Yuan et al. Used Ru / C and ni-mo-w (fhuds-2) as composite catalysts to reduce and degrade sulfate lignin. The results showed that the molecular weight and aliphatic hydroxyl content of the degradation products decreased significantly, the phenolic hydroxyl content increased, and the sulfur content of lignin decreased by 96%. Zhou et al. Used microwave-assisted molecular sieve (HSZ) solid acid as catalyst to degrade alkali lignin, dealkaline lignin and lignosulfonate. The effects of lignin structure and solvent on the yield of degradation products were studied. The results show that the catalysts have good catalytic effect in methanol, and the yield of degradation products can reach 57.4%, 82.9% and 70.9% respectively.
In addition, formic acid, sodium formate, isopropanol, 9,10-dihydroanthracene and other hydrogen supply solvents can also be used as hydrogen sources to effectively degrade lignin. For example, formic acid decomposes into CO2 and H2 at high temperature, which can provide hydrogen source for the system. Huang et al. Used formic acid as hydrogen source to reduce and degrade sulfate lignin. The results showed that with the increase of formic acid concentration, the molecular weight, polydispersity index and solid residue content of lignin decreased, and the degradation degree increased. Huang et al. Also used formic acid as hydrogen source and 10% Ni / zeolite as catalyst to partially reduce and degrade kraft lignin, resulting in high yield and low molecular weight compounds, and significant reduction of sulfur content in lignin. CANTAT group compared the effect of B (C6F5) 3 and Brookhart? Siri (Ⅲ) on the reduction and degradation of lignin by using hydrosilane as hydrogen source. It was found that the two catalysts could effectively convert lignin into phenol derivatives at room temperature, and Brookhart? Siri (Ⅲ) showed higher stability and selectivity than B (C6F5) 3, resulting in higher yield of aromatic compounds. Wu et al. Used ethanol / 1,4-dioxane as solvent, formic acid as catalyst and in-situ hydrogen donor as catalyst to degrade kraft lignin. The effects of solvent composition and reaction conditions on the conversion and yield of lignin were studied. The optimum process conditions were determined. The product contained 22.4% phenol monomers.
The degradation products obtained by reductive degradation method have good stability and high selectivity, which is an effective method for lignin degradation to prepare phenolic compounds. However, most of the methods require high temperature and pressure, and the cost of catalyst is high. Therefore, the selection and preparation of high catalytic activity, low-cost reduction catalyst is still the focus of current research.
2.2 oxidative degradation
Oxidative degradation is the main method of lignin treatment in modern industrial pulping and bleaching. This method can destroy the original chemical bond in the molecular structure of lignin by catalytic oxidation, and oxidize to get carbonyl compounds. Nitrobenzene oxidation and metal oxide / complex catalytic oxidation are two common methods of lignin degradation.
Nitrobenzene oxidation is the most classical method. In this method, nitrobenzene acts as an electron acceptor, and the whole reaction includes two processes: basic hydrolysis of ether bond and oxidation of side chain. Since Freudenberg found that basic nitrobenzene can oxidize lignin to produce a large number of aldehydes, this method has been used to study the molecular structure of lignin. After the oxidation of basic nitrobenzene, the types and yields of degradation products of lignin from different sources are different (Table 4).
The main degradation products of hardwood lignin are syringaldehyde and vanillin. The degradation products of softwood lignin are mainly vanillin, a small amount of p-hydroxybenzaldehyde and other oxidation products. The degradation products of herb lignin are mainly vanillin, eugenol and p-hydroxybenzaldehyde.
Metal oxide / complex catalytic oxidation is also a mature method for lignin degradation. In this method, the ether bond and carbon oxygen bond between the macromolecules of lignin are destroyed by the catalytic oxidation of metal oxides or complexes, so as to destroy the macromolecular structure and obtain the oxidation products. Hanson et al. Used vanadium complex to catalyze the degradation of β - O-4 lignin model compounds. The results show that the vanadium complex has the ability to catalyze the oxidation of alcohol hydroxyl to carbonyl, and the molecular weight of lignin is significantly reduced after degradation. Hdidou et al. Used the mixed oxides of Co3O4, Fe2O3 and cofeo as catalysts to oxidize and degrade lignin, determined the best process conditions, and obtained water-soluble oligomers with reduced molecular weight.
In addition to the above two methods, periodate oxidation, permanganate oxidation and electrochemical oxidation are also studied more oxidation degradation methods, and carboxylic acids, aldehydes and ketones can be obtained. Zhang et al. Studied the reaction mechanism of periodate oxidation degradation of lignin. It was found that the degradation process included the formation of quinone type intermediate, the increase of carboxyl content in lignin and the release of sugar after the chemical bond between lignin and carbohydrate was broken. Using nano alloy as catalyst, Movil Cabrera et al. Obtained aromatics with low molecular weight by electrochemical oxidation degradation of lignin, and studied the reaction mechanism of catalytic oxidation degradation of lignin. In addition, photocatalytic oxidation has the advantages of pollution-free and clean, which makes the research on the degradation of lignin widely concerned at home and abroad. Magallanes et al. Found that visible light catalytic oxidation degradation of lignin has high chemical selectivity, and the main product is β - O-4 bond cracking at room temperature and atmospheric pressure.
The oxidative degradation reaction conditions are mild, generally 150 ~ 250 ℃ can achieve better degradation effect, but the conventional chemical oxidation method has low degradation efficiency, low product selectivity and complex composition. Therefore, in order to improve the yield and selectivity of the product, good oxidation catalyst is still the main research direction.
2.3 water